Among conventional recording materials used for the recording layer of such a holographic recording medium as mentioned above are rewritable ones including photorefractive materials and write-once types including photopolymers.
With the latter write-once type recording materials, certain photosensitive components are consumed irreversibly by recording (exposure). The entire recording medium must therefore be prevented from exposure to light in external environment (ambient exposure) until the recording completes.
This requires, for example, that the recording be performed in an environment with little radiation of light at exposure wavelengths, or that a cartridge or the like having an excellent shielding property be used for protection.
In an environment with no radiation of light at exposure wavelengths, and with recording beams of visible light in particular, there is the problem that the recording must be performed in a darkroom and thus is unrealistic for a recording medium intended for storage systems.
The cartridge or other protection facilitates transportation, storage, and recording under normal environment, whereas the recording medium and system may get complicated in structure. In particular, this requires an openable and closable window for allowing passage of the light beams for recording and reproduction, aside from cost for providing the cartridge. Ambient exposure also occurs while the window is opened during recording. Moreover, depending on the physical relationship between the window and the recording layer, the angles of incidence, the positions of incidence, and the like of the beams are limited with a drop in the degree of freedom of the optical design. Furthermore, there is another problem of requiring a mechanical or electrical control mechanism for opening and closing the above-mentioned window in addition to the configuration necessary for ordinary recording and reproduction, with an increased size or cost of the recording medium.
As measures against the foregoing problems, it has been proposed to improve the optical characteristics of the photosensitive materials as described in Japanese Patent Laid-Open Publication No. Hei 7-181876.
This will not produce any essential improvement, however, since the recording mechanism and the shelf life of the data storage both are photochemical processes. More specifically, there is a trade-off such that the recording materials can be improved in recording sensitivity with a decrease in shelf life while damage from ambient exposure can be remedied with a drop in recording sensitivity.
A problem to be solved by this invention is to improve the trade-off between the recording sensitivity and the shelf life, and improve the shelf life of a recording medium that has an excellent recording sensitivity.